From the product publication “Sonderausgabe telcom report und Siemens Magazin Com: ISDN im Büro—HICOM” [Special edition of telcom report and Siemens' Com magazine: ISDN in the Office—HICOM], Siemens AG, Berlin and Munich, 1985, in particular from pages 14 to 31, a communication system designed for information switching, in particular for voice data switching, is known which supports a large number of features such as automatic callback, call pickup, call forwarding, etc. It is furthermore known from the 1991 product publication “HICOM 300 Networking”, reference number A31001-W-A30, issued by the company Siemens AG, how to link up communication systems into a—generally private—communication network and make features that are available locally on one communication system available throughout the network. The features are usually implemented across the network by a networking protocol, specific to the communication system, that is based on a timeslot-oriented transmission protocol.
This gives each subscriber of a communication system the possibility of activating or deactivating features, such as an automatic callback, on a network-wide basis. To do this the subscriber dials, for example, a feature-specific service code and the directory number of the terminal for which the relevant feature is to be activated or deactivated.
ITU-T (International Telecommunication Union) Recommendation H.450.9 describes a method for an implementation of the known “automatic callback” feature in packet-oriented networks according to the H.323 standard. Analogously to timeslot-oriented communication networks, a distinction is made here between two different cases:
1) automatic callback in the event of a “busy” destination subscriber (CCBS) and
2) automatic callback in the event of a “free” destination subscriber (CCNR).
A requirement of both variants is for the subscribers to be registered on the network via their respective terminals. Besides the two statuses “busy” and “free”, in packet-oriented networks employing the Internet Protocol—IP for short—there is another status in which the subscriber is neither “busy” nor “free” but is “not registered”. This is the case, for example, when an application implemented on the relevant terminal has not been launched or the terminal is inactive.